The present invention relates to magnetic sensors as applied to a magnetically active shaft structure, and more particularly, to non-contacting magnetoelastic torque transducers for measuring torque applied to a rotating shaft.
Measurement of torque is generally a primary consideration in systems utilizing rotating drive shafts. Determining torque applied to a rotating shaft in an accurate, reliable and inexpensive manner is a primary goal. For example, determining torque is critical in power steering systems of modern automobiles. In such systems, an electric motor assists the vehicle""s steering system in response to torque applied to the steering wheel by the driver. While advances have been made in power steering systems, there remains a compelling need for inexpensive torque sensing devices that are capable of continuous torque measurement over extended periods of time despite severe environmental and operating conditions.
Prior art torque measurement has been accomplished by using contact-type sensors directly attached to the shaft. One such sensor is a xe2x80x9cstrain gaugexe2x80x9d type torque detection apparatus, in which one or more strain gauges are directly attached to the outer peripheral surface of the shaft and a change in resistance caused by strain is measured by a bridge circuit or other well-known method. However, such contact-type sensors are relatively unstable and of limited reliability due to the direct contact with the rotating shaft. In addition, they are very expensive and are thus commercially impractical for competitive use on vehicle steering systems.
U.S. Pat. No. 6,145,387, issued to Garshelis, et al. on Nov. 14, 2000, which is hereby incorporated by reference, describes a magnetoelastic torque sensor, wherein a shaft of magnetostrictive, ferromagnetic material is endowed with axial regions of remanent, circumferential magnetization. Application of torque to such a shaft creates shear stresses within the magnetized regions, causing the direction of magnetization to shift from circumferential to helical, with a net axial magnetic component extending outwardly of the shaft. The axial magnetic field is proportional to applied torque, and is highly independent of environmental, thermal, and aging effects.
U.S. Pat. No. 6,128,964, issued to Sobel on Oct. 10, 2000, which is hereby incorporated by reference, describes a torque sensor with a polarized magnetic ring. When the shaft is torqued, the polarized ring generates an axial static magnetic field. A band of non-linear magnetic material surrounds the polarized ring. Fixed to the magnetic band, one or more solenoidal coils surround the shaft that are supplied with an alternating current of sufficient amplitude as to periodically, magnetically saturate the non-linear magnetic band. The magnetic field from the shaft is superimposed upon the periodic magnetic field from the coils to create an asymmetry in the saturation of the band. Changes in inductance of the coils due to the saturation of the magnetic band result in a voltage being induced in the coils. A phase-sensitive detector connected to coil windings supplies a signal corresponding to the magnetic field of the shaft which is indicative of the applied torque. When desired to distinguish the magnetic field generated by the magnetoelastic torque sensor from external, non-divergent, solenoidal magnetic fields (i.e. earth""s), two or more opposing magnetic active regions can be configured to produce one or more zones of axially divergent magnetic fields in response to torque.
Magnetic field sensors must be mounted proximate to the shaft to sense these divergent magnetic fields while rejecting solenoidal (non-divergent) interfering fields. In the prior art, such a magnetic gradiometer is comprised of two or more sets of oppositely-oriented magnetic field sensors located over each of the active regions of the shaft. If the axial orientation of each of these discrete magnetic field sensors is not parallel to the axis of the magnetoelastic shaft, this magnetic field gradiometer array will exhibit sensitivity to impinging, solenoidal magnetic fields. If the sensitivity of the magnetic sensor array does not exhibit symmetry in each axis, it will exhibit sensitivity to impinging, solenoidal magnetic fields. Thus, particular care is required in the fabrication of such a magnetic field sensor array so as to ascertain the matching and orientation of each discrete magnetic sensor in the array.
If the shaft is allowed to rotate independently of the magnetic field sensor, any angular variations in the magnetic field, due to physical or magnetic non-homogeneities in the magnetoelastic shaft, will result in a periodic signal appearing at an angular reference point on the magnetic field sensor. Conventionally, a multiplicity of sensors is positioned in an equiangular distribution around the shaft, and their outputs averaged, thus attenuating any signal resulting from rotation of the shaft. The amplitude of any remaining rotational signal is nominally inversely proportional to the number of discrete magnetic field sensors used. It is recognized that the cost and complexity of such a system increases with the number of magnetic field sensors, whereas reliability is reduced.
Accordingly, it is desired to provide a single magnetic field sensor that surrounds the shaft without variation in sensitivity along its circumference, and no sensitivity to interfering, isotropic magnetic fields.
Furthermore, it is desirable to sense, for each active region, the torque-related magnetic field over as broad an angular extent around the shaft as possible so as to increase the efficiency of the transfer function from magnetic field to electrical signal. In many applications, the selection of the shaft material may favor mechanical performance or cost over the magnetoelastic properties, thereby reducing the amplitude of the torque dependant magnetic field under a given stress level. In many of the prior art solutions, this necessitates the use of numerous discrete sensors, with corresponding additional cost.
Accordingly, there is a need for a magnetometer that can measure the magnetic field surrounding a rotating shaft without the requirement of multiple discrete sensors.
In view of the foregoing, it is an object of the present invention to provide a magnetoelastic torque sensor providing increased accuracy.
A second object of the present invention is to reduce the cost of manufacturing a magnetoelastic torque sensor.
Another object of the present invention is to reduce sensitivity of a magnetoelastic torque sensor to external, ambient magnetic fields.
A further object of the present invention is to reduce the strict homogeneous magnetic requirements of saturable magnetoelastic material used in magnetoelastic torque sensors.
According to the present invention, a flux-gate magnetometer torque sensor is provided having a rotatable shaft to which a torque force is to be applied, a sleeve of conductive foil affixed to the surface of the shaft over the magnetically active regions, a plurality of saturable magnetic wires or strips mounted to the rotatable shaft and parallel to an axis of rotation, sensor circuitry containing an oscillator for generating a signal, a divider coupled to the oscillator for dividing the frequency of the signal by two, a first and second coil each surrounding a different section of the rotatable shaft and having an input coupled to the divider output, a multiplier having inputs coupled to outputs of the first coil, the second coil, and the oscillator, and an integrator having an input coupled to the multiplier output and an output coupled to both outputs of the first and second coils, wherein the output voltage of the integrator corresponds to the torque being applied to the rotatable shaft.